The present invention relates to optical systems and, more particularly, to an optical system consisting, first, of a lens with adjustable focal length and an aperture plate whose opening is variable so as to maintain constant the amount of light leaving the lens and, secondly, of a light-meter device which is attached to the aperture plate adjustment device.
For those optical systems which are used especially in photographic or cinematographic cameras, it is desirable to indicate to the user of the system the limits within which an object must reside, in order to maintain a sharp image of the object in an image plane, as defined by the adjustment data provided by the system. It is known that the sharpness of an object image depends on the exactness of the focus adjustment and this, in turn, on the diameter of the permissible blur range in the image plane, through which a definite depth-of-focus range is defined in the object space. For pancratic lenses this depth-of-focus range becomes smaller as the focal length increases, therefore the danger arises that, with fixed focus adjustment, the diameter of the blurred object image exceeds the permissible diameter of the blur range. This danger is lessened if the lightness of the object is very great, whereby the amount of light leaving the optical system is kept constant by means of an aperture plate controlled by an exposure meter, which aperture plate is open only slightly if the object is very light.
Austrian patent document Nos. 329,375 and 240,620 describe constructions with whose help, depending on the adjusted focal length and the opening of the aperture plate, respectively, or depending only on the focal length, the system is focused so that the object is continually in the depth-of-focus area. Also, there are already cameras, for example, that are described in the Austrian patent document No. 280,779, which corresponds to U.S. Pat. No. 3,587,431, which monitor the focal length and the aperture value through coupling devices and which indicate the position of the object in reference to the depth-of-focus area in the viewfinder. However, cameras, which are provided with these devices, must have additional control elements, which tax the electronic switching measures. On the other hand the first-named focus-adjustment aids depend on subjective judgements of the operator and, therefore, do not necessarily function correctly at all times.